1. Field of the Invention
The present invention relates to a motor drive apparatus used in an injection molding machine and adapted to supply electrical energy to a motor included in an injection apparatus or the like.
2. Description of the Related Art
Generally, an injection apparatus and a mold clamping apparatus--which together constitute a motor-driven-type injection molding machine--each contain an AC servomotor; and in each case electrical energy is supplied from a motor drive apparatus to the corresponding AC servomotor in order to advance and retract a screw provided in the injection apparatus or a movable platen provided in the mold clamping apparatus (see, for example, Japanese Patent Application Laid-Open (kokai) No. 4(1992)-278321).
Incidentally, in an injection step of a molding cycle, an injection operation for advancing at high speed the screw accommodated within a barrel of the injection apparatus is performed, and in a mold clamping step of the molding cycle, a mold open/close operation for advancing and retracting at high speed the movable platen--which is provided in the mold clamping apparatus to support a movable mold--is performed. Similarly, another recurring operation is performed in order to move the screw in a measurement step, and yet another recurring operation is performed in order to move an ejector pin in an ejection step for ejecting a molded product.
As described above, the molding process of the injection molding machine involves an operation for moving a heavy member such as a screw or a movable platen at high speed during each molding cycle. Therefore, at the beginning of movement or during a period in which a motor is accelerated, drive current instantaneously flows through the motor in an amount several times that flowing during a constant speed period subsequent to the acceleration period, and at the end of movement or during a period in which the motor is decelerated, the motor instantaneously generates a considerable amount of regenerative current.
Therefore, in a conventional motor drive apparatus, the circuit must be designed such that, in consideration of excessive drive current (peak current) flowing during the acceleration period, a performance margin is provided, resulting in an increased size of the apparatus and increased cost. In addition, since the regenerative current generated during the deceleration period is consumed and dissipated in the form of heat by use of a regenerative resistor or the like, energy is consumed wastefully.